Abstract Malaysia's HIV epidemic is rapidly expanding with recent evidence suggesting accelerated sexual transmission, especially among men who have sex with men (MSM): 10% in 2008 to 54% in 2016. Biobehavioral surveys suggest high HIV prevalence in MSM being nationally (21.6%) and in Kuala Lumpur (43%). This growing epidemic in MSM is associated with condomless sex (CS), STIs, and high usage of substance (particularly amphetamine-type stimulants; ATS). In Malaysia, both substance use and homosexuality are criminalized, making MSM dually bear the burden of social stigma and discrimination, including in healthcare. Consequently, they are marginalized from traditional, venue-based HIV prevention services, which undermines recommendations by international agencies. To address stigma and discrimination, mHealth (mobile technology to promote health) strategies can deliver prevention messages and guide health decisions anonymously, and thus hold great promise for MSM, especially those using ATS. They can overcome barriers along the HIV prevention cascade that now includes pre-exposure prophylaxis (PrEP). Combined with strong training and mentoring, the proposed projects directly address this need through the conduct of two formative studies. First, information about MSM's preferences of various attributes of PrEP delivery programs, including segmentation of ATS-using MSM, will be assessed. Then, building on the first study, the candidate will develop and test an mHealth-based intervention to overcome barriers along the prevention continuum, including HIV testing, co-occurring sexual and drug risk behaviors and PrEP uptake among Malaysian MSM. Findings from these related projects will provide the candidate with the necessary skills and competencies to develop and test an innovative and tailored primary HIV prevention intervention for MSM. This K01 application for Dr. Shrestha is well-suited to advance the career of the candidate by virtue of his training in intervention design, delivery, and placement, his high productivity in public health research, and his evolving experience with research in the Malaysian context. Over 5 years, he will achieve new skills and experiences to achieve his professional goals and objectives to: 1) gain experience in decision science with advanced quantitative and qualitative skills to become an independent investigator in HIV, substance use, and mHealth intervention for vulnerable populations; 2) develop expertise in mHealth intervention development, testing, implementation, and dissemination that will address specific barriers to HIV prevention services in MSM, especially those with substance use disorders; and 3) develop an independent career path focusing on HIV prevention in MSM using mHealth strategies. To achieve these goals, he has assembled a stellar interdisciplinary team of mentors with expertise in HIV, substance use, intervention design, decision science, advanced biostatistics, and mHealth research. Under their guidance, he will complete relevant didactic work and attend seminars and conduct research to apply the skills needed to become an independent researcher in mHealth-based HIV prevention among MSM.